Research indicates that aircraft loss-of-control incidents represent the leading cause of fatalities both in air transport and general aviation operations. While the precipitating causes of vehicle upsets are diverse, incident findings suggest that a flight crew can become disoriented and incapable of responding to unusual attitudes and other envelope excursions. Several aspects of the interaction between the flight environment and the human perceptual system contribute to these difficulties: 1) the aircraft generates forces that can be misinterpreted as the effect of gravity, making it difficult to establish vertical orientation; 2) pitch and roll maneuvers often do not generate a sense of tilt with respect to gravity; 3) accelerations can induce an inaccurate sense of tilt; and 4) sustained rotations can impede the ability to sense angular motion. The effects of these aspects noted above may be more pronounced in environments where visual cues are deficient. Under circumstances when many of these elements are present, it may be difficult for a pilot to respond appropriately.
Generally speaking, normal flight may be impacted by vehicle movement and external forces (hazard or disturbance), causing poor situational awareness (distraction), spatial disorientation, and mode confusion, resulting in abnormal attitudes, abnormal trajectory, and loss of aircraft control, for example, a stall.
The human vestibular system, which contributes to balance and to the sense of spatial orientation, is the sensory system that provides the leading contribution regarding movement and sense of balance. The vestibular system comprises two components that detect rotation and translation: the semicircular canal system (three orthogonal semicircular canals), which indicates rotational movements; and the otoliths, which indicate linear accelerations. The vestibular system sends signals primarily to the neural structures that control eye movements, and to the muscles that allows one to sense an upright position. Movement of fluid within the horizontal semicircular canal corresponds to rotation of the head around a vertical axis, for example, the neck when doing a pirouette. The anterior and posterior semicircular canals detect rotation of the head in the sagittal plane, for example, when nodding, and in the frontal plane, for example, when cartwheeling. The movement of fluid pushes on hair cells that transduce the mechanical movement to electrical signals.
Experience from the vestibular system is called equilibrioception. It is mainly used for the sense of balance and for spatial orientation. When the vestibular system is stimulated without any other inputs, one experiences a sense of self-motion. For example, a person in complete darkness and sitting in a chair will feel that he or she has turned to the left if the chair is turned to the left. A person in an elevator, with essentially constant visual input, will feel she is descending as the elevator starts to descend. Although the vestibular system is a relatively fast sense used to generate reflexes to maintain perceptual and postural stability, compared to the other senses of vision, touch and audition, vestibular input is perceived with delay.
Because human senses are believed to be adapted for use on the ground, navigating by sensory input alone during flight can be misleading: sensory input does not always accurately reflect the movement of the aircraft, causing sensory illusions. These illusions can be extremely dangerous for pilots.
Illusions involving the semicircular and somatogyral canals of the vestibular system of the ear occur primarily under conditions of unreliable or unavailable external visual references and result in false sensations of rotation and/or motion. These include the leans, the graveyard spin and spiral, and the Coriolis illusion.
The leans is the most common illusion during flight, and is caused by a sudden return to level flight following a gradual and prolonged turn that went unnoticed by the pilot. The reason a pilot can be unaware of such a gradual turn is that human exposure to a rotational acceleration of 2 degrees per second squared or lower is below the detection threshold of the semicircular canals. Leveling the wings after such a turn may cause an illusion that the aircraft is banking in the opposite direction. In response to such an illusion, a pilot may lean in the direction of the original turn in a corrective attempt to regain the perception of a correct vertical posture.
The graveyard spin is an illusion that can occur to a pilot who enters a spin. For example, a pilot who enters a spin to the left will initially have a sensation of spinning in the same direction. However, if the left spin continues the pilot will have the sensation that the spin is progressively decreasing. At this point, if the pilot applies right rudder to stop the left spin, the pilot will suddenly sense a spin in the opposite direction (to the right).
If the pilot believes that the airplane is spinning to the right, the response will be to apply left rudder to counteract the sensation of a right spin. However, by applying left rudder the pilot will unknowingly re-enter the original left spin. If the pilot cross-checks the turn indicator, he would see the turn needle indicating a left turn while he senses a right turn. This creates a sensory conflict between what the pilot sees on the instruments and what the pilot feels. If the pilot believes the body sensations instead of trusting the instruments, the left spin will continue. If enough altitude is lost before this illusion is recognized and corrective action is not taken, impact with terrain may occur.
The graveyard spiral is more common than the graveyard spin, and it is associated with a return to level flight following a prolonged bank turn. For example, a pilot who enters a banking turn to the left will initially have a sensation of a turn in the same direction. If the left turn continues (for more than about 20 seconds), the pilot will experience the sensation that the airplane is no longer turning to the left. At this point, if the pilot attempts to level the wings this action will produce a sensation that the airplane is turning and banking in the opposite direction (to the right). If the pilot believes the illusion of a right turn (which can be very compelling), he will re-enter the original left turn in an attempt to counteract the sensation of a right turn.
An illusion caused by the coriolis involves the simultaneous stimulation of two semicircular canals and is associated with a sudden tilting (forward or backwards) of the pilot's head while the aircraft is turning. This can occur when tilting the head down (to look at an approach chart or to write on the knee pad), or up (to look at an overhead instrument or switch) or sideways. This can produce an overpowering sensation that the aircraft is rolling, pitching, and yawing all at the same time, which can be compared with the sensation of rolling down a hillside. This illusion can make the pilot quickly become disoriented and lose control of the aircraft.
Somatogravic illusions are caused by linear accelerations. These illusions involving the utricle and the saccule of the vestibular system are most likely to occur under conditions with unreliable or unavailable external visual references.
Accordingly, it is desirable to provide an alert to the pilot when current flight parameters produce conditions where pilot disorientation is likely. Furthermore, other desirable features and characteristics of the exemplary embodiments will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.